Image sequences may be transmitted between video displays for passive capture by a camera equipped device. Such transmissions may also encode data within the video image sequence. Data bits may also be presented in an image format, e.g. an animated rectangular grid pattern of black and white cells, to effect data transfer between a video display and a separate camera equipped device passively observing that display.
Transmitting data over such a display-to-device channel is complicated by the potential for (a) dropped frames, (b) a partially refreshed display when sampling, and (c) general environmental noise. For image sequences that loop continuously it can also be difficult to detect the start and end of the image sequence.
Consider a display refreshing at D Hertz (Hz), containing encoded data animating at C Hz. The camera frame rate is R Hz. The following error cases may arise:—
Case (a): Dropped Frames.
The Nyquist limits of the display and camera are D/2 and R/2 respectively. If C>D/2, D>R/2, or C>R/2 then it is likely that the camera will fail to image a subset of frames, i.e. frames will be “dropped”.
The frequency of frame dropping is proportional to C/D, D/R and C/R but in practice C/R is the significant factor on typical equipment. Frames may also be dropped due to external factors such as misregistration of an encoded image or the environment (e.g. disruption of the line of sight between display and camera).
Case (b): Partial Refresh.
If a (raster) video display is refreshed at D Hz from top to bottom by a scanline sweep, it is possible that, at the instant when the camera samples the image, the display is only partially refreshed i.e. half the image data displayed is from the previous frame, and half is from the current (new) frame. For practical purposes, the rate at which this phenomenon occurs is proportional to the size of the image frame on the screen (typically affecting approximately 1% of frames), with insignificant correlation to D and R on typical equipment.
Case (c) Garbled Frame.
Occasionally, displayed frames will become garbled e.g. due to the camera sampling its image in the ‘idle’ (dim) phase of the display refresh cycle, or due to environmental factors. The rate of this phenomenon is typically constant and independent of D, R and C.
Furthermore, when transmitting a continuously looped sequence of images, it is desired to ascertain SYNCHRONIZATION conditions relating to the sequence. For a sequence of frames that are displayed in continuous cycle, it is desirable to signal out-of-band which of these frames has the attribute of being the first frame (“start frame”) of the sequence to enable the correct ordering of data during decoding.